So far the market of UHF RFID transponders has been divided into cheap labels and more expensive rigid tags that are applicable also on metal platform. The typical rigid transponders are large and expensive and thus applicable only on big and expensive items. Some small rigid tags have entered the market but with significantly lower performance than the big ones.
On today's market, there are already some small metal-mountable RFID transponders available. In this context, “small” can be defined as the footprint area of the transponder being less than 10 cm2 and the maximum dimension being less than quarter of the wavelength (86 mm @ 867 MHz). Generally, the problem with these small tags is that making a transponder small in size always leads to some compromises. Practically, these compromises reduce the read range which is the key parameter for evaluating the reliability and performance of a passive RFID system.
First of all, the radiation efficiency of the transponder is reduced by the size limitation. This is basically a physical fact that cannot be fully overcome, only minimized by e.g. optimal materials. But with very small antennas, the poor impedance match between the microchip and the antenna typically causes even greater part of the loss in read range than the reduced radiation efficiency. The conjugate impedance coupling between the antenna and the microchip that is needed for optimal operation typically cannot be achieved with a very small antenna and the current methods. The most typical solution with very small tags is to provide only reactive coupling, which is a partial solution. In the case of more complex radio systems with antennas, such as mobile devices and base stations, this impedance match needed is often achieved using discrete components (capacitors and inductors) or microstrip elements. For small RFID tags, microstrip elements are a way too large. Discrete components would be small enough, but they are out of question for cost and processability reasons.
The small metal-mount transponders existing on today's market have somewhat compromised performance. Additionally to the low radiation efficiency of the small antenna, their read range is remarkably decreased by the poor impedance match between the microchip and the antenna.
There are several different types of RFID transponder: passive, semipassive (or battery assisted passive) and active, as well as those to which a connection can be made inductively, capacitively, or with the aid of a radio-frequency radiation field. Passive transponders generate the electrical energy they need from the RF field aimed at them. In active and semipassive transponders, there is a separate battery or other power supply. Inductively connected RFID and remote sensor systems typically operate at frequencies of 100-125 kHz or 13.56 MHz.